a. Field of the Invention
This invention relates to a system and method for assessing the formation of lesions in tissue in a body. In particular, the instant invention relates to a system and method for assessing the formation of lesions created by one or more electrodes on a therapeutic medical device, such as an ablation catheter, in tissue, such as cardiac tissue.
b. Background Art
It is generally known that ablation therapy may be used to treat various conditions afflicting the human anatomy. One such condition that ablation therapy finds a particular application is in the treatment of atrial arrhythmias, for example. When tissue is ablated, or at least subjected to ablative energy generated by an ablation generator and delivered by ablation catheter, lesions form in the tissue. More particularly, electrodes mounted on or in ablation catheters are used to create tissue necrosis in cardiac tissue to correct conditions such as atrial arrhythmia (including, but not limited to, ectopic atrial tachycardia, atrial fibrillation, and atrial flutter). Arrhythmia can create a variety of dangerous conditions including irregular heart rates, loss of synchronous atrioventricular contractions and stasis of blood flow which can lead to a variety of ailments and even death. It is believed that the primary cause of atrial arrhythmia is stray electrical signals within the left or right atrium of the heart. The ablation catheter imparts ablative energy (e.g., radiofrequency energy, cryoablation, lasers, chemicals, high-intensity focused ultrasound, etc.) to cardiac tissue to create a lesion in the cardiac tissue. This lesion disrupts undesirable electrical pathways and thereby limits or prevents stray electrical signals that lead to arrhythmias.
One challenge with ablation procedures is in the assessment of the lesion formation as a result of the application of ablative energy to the tissue. For example, it may be difficult to determine whether a particular area of tissue has been ablated or not, the extent to which ablated tissue has been ablated, whether a lesion line is continuous or has gaps therein, etc. Lesion formation has typically been fairly crudely assessed using any one of a number of different empirical techniques.
One such technique depends on a subjective sense for catheter contact combined with RF power settings, for example, and the duration the electrode spends in contact with the tissue. Another technique employs temperature sensing. Ablation generators and their ablation catheters monitor temperature, but with the advent of saline cooled catheters, temperature has gone from an index of catheter temperature (and less directly an index of tissue temperature), to a nearly useless index primarily reflecting irrigant saline flow. A further method relies on ablation catheter electrogram signals. RF ablated myocardium demonstrates poor depolarization wavefront conduction and thus local electrogram amplitude reduction and morphology changes are sometimes, but not consistently, observed. Accordingly, the assessment of lesion formation has ordinarily no direct objective basis.
The inventors herein have recognized a need for a system and method for assessing or the formation of lesions in tissue that will minimize and/or eliminate one or more of the above-identified deficiencies.